ABSTRACT Medulloblastoma is the most common malignant brain tumor that afflicts children. Despite therapy with surgery, radiation and chemotherapy, outcomes of these highly toxic treatments are sub-optimal with significant long-term morbidity. Medulloblastoma consists of 4 distinct molecular subgroups (Wnt, Sonic Hedgehog, Group 3 and Group 4). Among these, Group 4 medulloblastoma is the most common subgroup but its underlying biology is the least characterized. We have recently demonstrated a critical role for Enhancer of Zeste Homolog 2 (EZH2) in Group 4 medulloblastoma. EZH2 is the catalytic core protein of the PRC2 chromatin-remodeling complex, which catalyzes the trimethylation of histone3 lysine27 (H3K27me3) and mediates epigenetic silencing of genes involved in cell fate decisions, differentiation and cancer. We demonstrated that EZH2 is overexpressed in Group 4 medulloblastoma patients, regulates the proliferation and self-renewal capacity of medulloblastoma cells and that the H3K27me3 mark is enriched in Group 4 medulloblastoma patients associated with adverse outcomes. However, the mechanisms underlying EZH2 mediated control of medulloblastoma tumorigenesis are poorly understood. Our preliminary data demonstrates that EZH2 suppresses expression of key regulators of neuronal differentiation and promotes transformation of neural stem cells, raising the intriguing possibility that aberrant EZH2 expression enforces a neuronal differentiation block and maintains pluripotent state in neural and tumor stem cells. The exact gene expression programs regulated by EZH2 in medulloblastoma are unknown and the impact of abnormal EZH2 expression in the cerebellum is undetermined. We hypothesize that EZH2 mediates medulloblastoma tumorigenesis by inhibiting differentiation of cerebellar stem cells and maintaining a pluripotent state. Our objective is to examine the biological impact of EZH2 on medulloblastoma tumorigenesis and test potential novel therapeutic molecules targeting EZH2. To pursue our hypothesis we will first investigate the proposition that EZH2 suppresses gene expression of neuronal differentiation programs in medulloblastoma by altering chromatin occupancy of the H3K27me3 histone core at key promoters. Next we will pursue concept that aberrantly increased EZH2 expression in Group 4 specific cerebellar stem cells will inhibit differentiation and induce tumor formation in the murine cerebellum using a novel mouse model of medulloblastoma. Finally we will test three clinically relevant inhibitors in vivo using patient derived xenograft models of medulloblastoma. Successful completion of the proposed work will determine the role of EZH2 in medulloblastoma and establish the potential of therapeutically targeting this enzyme in clinically relevant in vivo models.